(1) Field of the Invention
The present invention relates to a fixing device and an image forming apparatus using the fixing device, and in particular to a technology for extending the life of a fixing belt in a fixing device, the fixing belt including a resistance heat layer and electrode layers for supplying power to the resistance heat layer.
(2) Description of the Related Art
Among conventional image forming apparatuses such as printers, there are some that have adopted a fixing device that uses a fixing belt containing a resistance heat layer, the fixing device being able to conserve more energy than a fixing device that uses a halogen heater as the heat source, as disclosed in, for example, Japanese Patent Application Publication No. 2009-109997.
FIG. 11 is a perspective view illustrating an example of the structure of such a fixing unit 500.
As shown in FIG. 11, the fixing unit 500 includes a fixing belt 554, a pressure roller 550, a pressurizing roller 560, and a pair of power supply rollers 570 connected to an AC power source.
The fixing belt 554 is a cylindrical, flexible and deformable belt provided with a resistance heat layer 554b, and on the circumference of the fixing belt 554 at the two end portions in the width direction (Y axis direction), electrodes 554e are respectively formed on the resistance heat layer.
The pressure roller 550 is composed of a cored bar 551 and an elastic layer 552, wherein the cored bar 551 is covered with the elastic layer 552, and the pressure roller 550 is movably inserted in the inside of a running path of the fixing belt 554.
The pressurizing roller 560 is provided over the running path of the fixing belt 554, and presses the pressure roller 550 via the fixing belt 554, thereby forming a fixing nip therebetween.
Also, the pressurizing roller 560 receives a driving force from a driving motor (not illustrated) and rotates in the direction indicated by the arrow P shown in FIG. 11. This driving force is conveyed to the pressure roller 550 via the fixing belt 554, and causes the fixing belt 554 and the pressure roller 550 to rotate passively in the direction indicated by the arrow Q shown in FIG. 11.
The pair of power supply rollers 570 are structured to contact with the respective electrodes 554e of the fixing belt 554 from over the running path of the fixing belt 554, and press the electrodes downward in FIG. 11. This causes power to be supplied to the resistance heat layer 554b of the fixing belt 554.
When the fixing belt 554 is driven to move cyclically and power is supplied to the electrodes 554e, power is supplied to the resistance heat layer 554b of the fixing belt 554, and then the whole resistance heat layer 554b is heated.
In the above state, the fixing belt 554 is only in contact with the fixing nip 530 and the pair of power supply rollers 570, thus the fixing nip 530 is effectively heated, and a toner image having been formed on a recording sheet (not illustrated) is fixed on the recording sheet by the heat and pressure when the recording sheet passes through the fixing nip 530.
However, when the fixing unit 500 is driven, the electrodes 554e of the fixing belt 554 are deformed as they receive pressures from both the pressurizing roller 560 and the pressure roller 550 repeatedly in the fixing nip 530. Thus a problem of the conventional fixing device is that a peel-off is easy to occur and the life of the fixing belt 554 is short.